RE: Battery alternative...
Specifically, KAN1050 and CBP1050 cells are drop-in NiMH replacements for the KR600AE NiCds. They give you better performance and double the flying time, for the same weight.
If you want to save some weight, you might be able to get away with the CBP650 NiMH cells, which are much smaller than any of the aforementioned cells. You'd be pushing them pretty hard, and you wouldn't get any additional flying time over the NiCds.
Personally, I'd get either of the 1050 cells. The Tipsy Nipper is designed for that size/weight cell.